488 COMPARATIVE ANATOMY 



the proximal end of which the vasa deferentia open. In Crocodiles 

 the free portion is relatively longer and the groove deeper than in 

 Chelonians. 



In many Birds a copulatory organ is present, formed on a 

 similar plan to that of Crocodiles. It is well developed amongst 

 the Ratitae and Lamellirostres, and in many other Birds can be 

 recognised in a rudimentary condition. In Struthio it resembles 

 that of the Crocodile, except that the distal free portion is longer; 

 it is grooved above, encloses cavernous tissues, and is supported by 

 a fibrous body, bifurcated at the base. In Dromseus and Rhea there 

 is an aperture at the apex of the penis leading into an elongated 

 and curved blind sac, in which is a furrow, lined by cavernous 

 tissue, continuous with the groove on the dorsal side of the organ. 

 In the Duck and Swan the spiral penis is essentially similar to that 

 of Dromseus and Rhea. The absence of the blind sac in the 

 Ostrich is probably a secondary modification. A clitoris is present 

 in the female of the above-mentioned Birds. 



The penis of Monotremes may be best understood by 

 imagining a hypothetical form intermediate between it and that of 



FIG. 367. TRANSVERSE SECTION OF THE CLOACA OF A CHELONIAN. Slightly 



diagrammatic. (After Boas.) 



"it 



;'. fibrous body : r, seminal furrow, bounded by cavernous -issue ; r, wall of 



cloaca. ^ 



Crocodiles and Chelonians (Figs. 368, B). We must suppose that 

 a sac-like outgrowth into which the ureters and vasa deferentia open 

 has become developed from the ventral cloacal wall at the base 

 of the penis, the groove in which has become converted into a 

 canal. The Monotreme condition is reached by the sac elongating 

 to form a urinogenital canal, into the distal end of which the 

 urinary and genital ducts and the bladder open (c, D). The penis 

 consists of an unpaired fibrous body enclosing the seminal 

 canal, and is only loosely surrounded by the mucous membrane 

 of the cloaca, so that it can be protruded from and retracted into 

 a sheath. In Echidna, cavernous tissue is present in the apex 

 or glans; and in Ornithorhynchus the apex is bifurcated and 

 covered with soft spines, the seminal canal or urethra opening 

 in each case by numerous fine canals situated on papillae. A 

 clitoris is present in the female of all Mammals. 



In Marsupials (Fig. 369, A), the penis-sheath opens on to the 

 surface of the body, below the anus ; the opening of the urino- 

 genital canal into the cloaca has become closed, and is continuous 



